Spherical Active Carbon And Process For Producing The Same

ABSTRACT

An active carbon produced from an infusible carbonaceous material as a raw material, which active carbon when used for prevention of automobile fuel evaporation, etc., would not cause any trouble attributed to dust generation, exhibiting reduced pressure loss; and a process for producing the same. There is provided a spherical active carbon produced from an infusible solid carbon material as a raw material, wherein providing that x represents an average particle diameter (mm) and y an MS hardness (%), when x is in the range of 0.5 to 20, y is ≧100×(1-0.8×1.45 (0.3-x) ). This spherical active carbon can be produced by a process comprising mixing a carbon material with a carbonizable binder; extruding the mixture into a strand form; carrying out rolling granulation so as to obtain a spherical form; rendering the same infusible under appropriate conditions corresponding to the particle size; and performing carbonization and thereafter activation under conditions appropriately restricting the contact with activation gas.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a spherical activated carbon. Moreparticularly, the present invention relates to a spherical activatedcarbon having high hardness produced from an infusible solid carbonmaterial as a raw material.

BACKGROUND OF THE INVENTION

An activated carbon, which has an excellent ability of adsorbing varioushazardous substances and offensive odor substances, has beenconventionally used as an adsorbent in a variety of fields for bothhousehold and industrial purposes. Such activated carbon is used invarious forms such as powder and pellet forming materials, depending onthe applications.

In recent years, since the use of the activated carbon has beenincreasingly expanded, the performance required for the activated carbonhas become strict according to the use of the carbon. For example, anapplication of the activated carbon to a filter for removing automobilefuel vapor has been advanced. However, when the automobile is equippedwith an activated carbon filter, strict control of dust generationprevention is required because adverse effects occur, that is, thefilter is exposed to a long-term vibration to likely cause dustgeneration, and once dust generation occurs, fine particles are mixedinto the exhaust system to cause trouble.

Furthermore, a superior low dust generation property is desirable in thesame way, such as the cases where the activated carbon is used forabsorbing hazardous substances in a clean room for producing drugs andthe like; where the activated carbon is used for absorbing hazardoussubstances inside or in their vicinity of precision equipment and anelectronic device which dislike dust, for example, for absorbingsubstances which exert an influence on a hard disk of computer and thelike; or where the activated carbon is used for a constant or higherflow rate of a gas, for example, used for a pressure swing system gasseparation device.

Moreover, in the applications accompanied by circulation of a gas suchas the filter removing automobile fuel vapor, it is also practicallyimportant that the pressure loss is small, and high absorption abilityis naturally required in all applications.

For the above-mentioned applications, there is a method of using anactivated carbon in a pulverized or a pellet form. However, because ofedge parts in pulverized or a pellet form, it is difficult toconsiderably reduce the dust generation attributed to a defect infilling into a container and a defect of the edge parts in the casewhere vibration is given over a long time when in use. In addition, thepressure loss tends to increase due to the irregular filling. Meanwhile,for example, if a forming material and the like in a honeycomb form isused, the problem of the dust generation is alleviated, however, theabsorption ability tends to be decreased because the surface area maynot be secured to the degree that the activated carbon in a pellet or apowder form is filled.

Therefore, a spherical form is desirable in order to satisfy theabove-mentioned required performance. A spherical activated carbon wouldnot cause dust generation, accompanied by fracturing when filled into acontainer, because of absence of edges in the form, and would notincrease drift and pressure loss due to the irregular filling. Moreover,since the spherical activated carbon has an excellent flowability whenit is used by fluidizing, it is easy to fill into a container having acomplicated form and it is suitably used to treat a liquid by a systemin which an activated carbon is fluidized.

There have been conventionally known a variety of spherical activatedcarbons and production methods thereof. When the spherical activatedcarbons are broadly classified by production methods, there are thefollowing two methods: (1) a method in which a liquid or fused materialis dispersed in a dispersion medium such as water to produce a sphericalparticle and then performing carbonization and activation, and (2) amethod in which a raw material powder and a binder are subjected torolling granulation to obtain a spherical form and then performingcarbonization and activation.

As a method of performing carbonization and activation after a rawmaterial is dispersed in a dispersion medium such as water to produce aspherical particle, in Patent Reference 1 (Published Examined PatentApplication No. Sho 50-018879) and Patent Reference 2 (PublishedUnexamined Patent Application No. Sho 55-113608), there is described amethod of producing a spherical carbon or a spherical activated carbonby fusing a pitch-based raw material, and dispersing and granulating theresultant material followed by infusibilization, carbonization andactivation. In addition, in Patent Reference 3 (Published UnexaminedPatent Application No. Hei 03-030834), Patent Reference 4 (PublishedExamined Patent Application No. Sho 46-41210) and Patent Reference 5(Published Unexamined Patent Application No. Sho 50-51996), there aredescribed a method in which a raw material powder and a binder areformed to a spherical form and then carbonization and activation areperformed, and an activated carbon obtained by the method.

Patent Reference 1: Published Examined Patent Application No. Sho50-018879

Patent Reference 2: Published Unexamined Patent Application No. Sho55-113608

Patent Reference 3: Published Unexamined Patent Application No. Hei03-030834

Patent Reference 4: Published Examined Patent Application No. Sho46-41210

Patent Reference 5: Published Unexamined Patent Application No. Sho50-5199

Non-patent Reference 1: Gas World Coking Section, 111 (1939) p. 106-111

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An activated carbon having a high hardness is readily obtained by amethod in which a raw material in a liquid state is dispersed in adispersion medium to form a spherical form under the operatingconditions as disclosed in Patent Reference 1 and Patent Reference 2. Inthis case, however, since it is required that the mixture is a liquidstate at the stage of dispersing the raw material in the dispersionmedium, a fusible raw material such as petroleum pitch is required to beused as a raw material, and a commonly-used carbon material such ascoconut shell and typical coal cannot be used as a main raw material.Moreover, normally the fluidity is increased by using a solvent,however, in this case, the process becomes complex because an extraprocess is required to remove the solvent.

Further, since these methods are difficult to maintain large particlesstably in a dispersion medium, it is generally difficult to produceindustrially a spherical activated carbon having a diameter of 1 mm ormore. In addition, when pitch and resin are formed to a spherical formand then are infusibilized and carbonized, the resultant spherical pitchand resin particles are closely adhered to each other, and thus a gasfor infusibilization and carbonization is difficult to reach the insideof the particles if the particle diameter becomes larger and thevolatile portions generated are difficult to be removed, thereby makingit difficult to perform homogeneous infusibilization and carbonization.For this reason, in general, these methods have limitations in thesurface area and absorption ability and are likely to cause thedegradation due to the structural difference between the surface and theinside.

Meanwhile, in Patent Reference 3, Patent Reference 4 and PatentReference 5, there are described a method of producing a sphericalactivated carbon by forming a raw material powder and a binder to aspherical form and then performing carbonization and activation or anactivated carbon produced by the production methods thereof. Accordingto these methods, a large activated carbon having a diameter ofapproximately a few mm may be produced by using an infusible solidcarbon raw material such as coconut shell charcoal and coal, and theactivation may be performed to the inside of the carbon because manymicrospores are contained even at the time when the spherical carbon isformed. However, the spherical activated carbon produced by thesemethods adversely has disadvantages in that the bulk density isdifficult to be increased, the hardness is low and the dust generationproperty is low.

In these precedent references, it is described that the activated carbonof each invention has a high hardness and low dust generation property.However, the test results by the inventors of the present inventionshowed that the dust generation property of these activated carbons wasnot small enough for the strict requirement required in the applicationspreviously mentioned.

In light of the above-mentioned situations, the object of the presentinvention is to provide a spherical activated carbon which is producedfrom an infusible solid carbon material as a raw material, would notcause any trouble attributed to dust generation when used for preventionof automobile fuel evaporation and has a small pressure loss.

Means to solve the Problems

As a result of earnest studies to achieve the above-mentionedobjectives, the present inventors have found that a spherical activatedcarbon having a high hardness may be produced from a commonly-used anduseful raw material such as coconut shell and coal by a processcomprising blending a raw material carbon powder with a binder,extruding the resultant mixture into a strand form, carrying out rollinggranulation to obtain a spherical form, infusibilizing the same underappropriate conditions, and performing carbonization and thereafteractivation under conditions appropriately restricting the contact withan activation gas, and the present inventors have completed the presentinvention.

That is, the present invention is a spherical activated carbon having aconstant or more hardness which is produced by an infusible solid carbonmaterial as a raw material.

Effects of the Invention

The present invention may provide a spherical activated carbon producedfrom a commonly-used and useful carbon material such as coconut shellcharcoal and coal as a raw material, which activated carbon has smalldust generation, a low pressure loss and a relatively large particlediameter when in use. In addition, the present invention may provide amethod of producing a spherical activated carbon having small dustgeneration and a relatively large particle diameter readily andindustrially.

BEST MODE FOR CARRYING OUT THE INVENTION

An activated carbon in the present invention is referred to as aspherical activated carbon produced from an infusible solid carbonmaterial as a main raw material. Herein, “infusible” means that a rawmaterial itself will not be fused to a liquid under conditions where itis granulated and infusibilized. In other words, a carbon material usedas a raw material of the present invention has a melting point ordecomposition point of 300° C. or more. Further, a carbon material meansthat the main ingredient comprises carbon and typically, is referred toas a material which is made up of carbon atoms comprising 60% by weightor more of the total weight after drying to remove water. Moreover, “asa main raw material” means that 50% by weight or more, preferably 70% byweight or more of the carbon content before infusibilization andcarbonization is derived from the solid carbon material.

As an infusible solid carbon material used as a raw material of anactivated carbon of the present invention, there may be mentionedcharcoal, coconut shell charcoal, various coals and various materialssuch as anthracite, bituminous coal and the like. However, since anactivated carbon which is easily available and has various propertiesmay be produced, coconut shell charcoal and coal are preferable. Amongthem, coconut shell charcoal is especially preferable in that anactivated carbon, which contains no hazardous impurities, is easilycommercially available and has a suitable fine-pore structure, isreadily manufactured.

The activated carbon of the present invention is a spherical activatedcarbon. Herein, “spherical” is referred to as a form having no sharpedges, which is different from a cylindrical pellet and pulverizedgranular activated carbon. Because of having a form having no such sharpedges, an activated carbon of the present invention is preferable inthat it may suppress the defect caused by vibration and collision withother particles and dust generation caused by the defect. Moreover, anactivated carbon of the present invention is preferable in that apressure loss is likely constant without localization and drift isunlikely occur because such form is regularly filled. Herein,“spherical” may be a form not having the above-mentioned sharp edges.Among forms having no edges, it is preferable to be more close to a truesphere. Specifically, the ratio of long radius to short radiuspreferably is 1.0 to 2.0, and more preferably 1.0 to 1.5.

In order to prevent trouble caused by dust generated from an activatedcarbon in using the activated carbon, it is preferable that a hardnessof a spherical activated carbon is high. Typically, a hardness of anactivated carbon is represented by a hardness measured by a methodspecified by JIS K1474 (hereinafter, abbreviated as JIS hardness).However, conventionally, if the activated carbons have been used for theapplications required for suppressing dust generation as mentionedabove, the JIS hardness typically exceeds 98%. Therefore, if theseconventional activated carbons and an activated carbon which showsclearly less dust generation than conventional carbons and high dustprevention are compared in JIS hardness, these differences are notclearly observed. Accordingly, it is difficult to evaluate whether anactivated carbon may highly prevent high dust generation using JIShardness as an index. Consequently, as a result of various studies tofind a hardness measuring method which may clearly reflect the degree ofdust generation, the present inventors have found that microstrengthhardness (hereinafter represented by an MS hardness) well agrees withthe degree of dust generation caused by the defect when in use, andadopted the MS hardness for evaluating an activated carbon of thepresent invention as an index.

The MS hardness measuring method in the present invention is a method inwhich the method used for measuring the hardness of coal is adjusted soas to adequately evaluate the particle diameter of an activated carbonto be covered by the present invention. The measuring method issummarized as follows. That is, 10 steel balls are added into a steelpot having an inside diameter of 25.4 mm and a length of 304.8 mm andfurther 5 g of a dried, granular activated carbon was added and sealed.The steel pot is attached to a measuring device and rotated at avelocity of 25 rpm for 40 minutes. After that, a sample is taken out andthe steel balls are removed, and subsequently the sample is sieved in avibrator for 5 minutes using a screen with an opening of 0.3 mm. Thepercentage, which is represented by the ratio of the sample thatremained on the sieve to the sample that was initially added to thesteel pot, is determined as the MS hardness.

Meanwhile, for the devices and the like which are not described in theabove measuring method, measurements are carried out in accordance witha method described in Non-patent Reference 1 (Gas World Coking Section,111 (1939) p. 106-111).

Since the above-mentioned MS hardness measuring method measures aremaining ratio after pulverizing activated carbons with steel balls andpassing fragments through a particular screen, even if the materials tobe measured are the same, the MS hardness is measured as a low value aslong as the particle diameter at the starting time of measurement issmall, and inversely is measured as a high value if that is large. Thatis, when the activated carbons originally have a larger particlediameter even if they are of the same material, the high values of thehardness are measured because they are not passed through the screeneven if they are divided into fragments. When they originally have asmaller particle diameter, low values of the MS hardness are measuredbecause they are passed through the screen even if they are not dividedinto fragments. Accordingly, when the hardness of an activated carbon isrepresented by the MS hardness, it is represented by a function of aparticle diameter in order to reflect the inherent material hardness. Asa result of various studies, the inventors have found out that,regarding the MS hardness of spherical activated carbons which aremanufactured by almost the same method and have almost the samepractical hardness, when the average particle diameter is represented byx (mm) and the MS hardness by y (%), the relationship of followingequation (I) holds between x and y.Y=100×(1−0.8×a ^((0.3-x)))  (I)

The above equation is empirical and the effects are as follows. First,the conditions to be satisfied by the MS hardness are reviewed. In thecase where the particle diameter to be measured is homogeneous and thescreen opening is smaller than the particle diameter x, the MS hardnessis forced to be zero because particles to be measured are passed througha screen without any pulverizing. Inversely, if x is considerablylarger, the MS hardness has to be approaching 100% because particles tobe measured are not easily passed through a screen even afterpulverizing.

At the same time, when the equation (I) is described in a general form,it is represented by the following equation (II).Y=100×(1−c×a ^((b-x)))  (II)In the equation (II), a^((b-x)) represents a ratio of fragments having adiameter passing the screen after pulverizing at the time ofmeasurement. b is an opening (mm) of a screen, and b equals 0.3 becausea screen with an opening of 0.3 mm is used in the above measurements. Incase of X=b, a^((b-x))=2 and a^((b-x))=0 if x is sufficiently large.Therefore, if c=0.1, the conditions to be theoretically satisfied by theMS hardness are satisfied in the absence of the above-mentioned particlediameter distribution to be measured. However, since an activated carbonhas a particle diameter distribution and clogging happens even when theparticle is smaller than the screen opening and parts remain on thescreen, even in the case of a spherical carbon having an averageparticle diameter of 0.3 mm, the MS hardness will not be zero. In thisway, the coefficient c of the equation is to adjust deviations from theideal relationship, and as a result of investigating the relationshipbetween the particle diameter and the MS hardness by measuring aspherical activated carbon of the present invention produced by the samemethod, which has various particle diameters in the range in which x is0.5 or more and 20 or less, it is empirically concluded that C=0.8.

In the equation (I), if the value of a is large, the MS hardness in thesame particle diameter becomes higher, and if the value of a is small,the MS hardness becomes lower. Therefore, it may be said that a is anindex representing an absolute hardness of an activated carbon material.By judging from the measuring method of the MS hardness, the MS hardnesswill be 100% irrespective of the particle diameter if the particle to bemeasured is not at all pulverized and the particle diameter to bemeasured is homogeneous and the diameter is larger than the screenopening. In the above equation (I), as a increases, y approaches to 100%and therefore in this respect the equation (I) satisfies the conditiondesired by the MS hardness.

A spherical activated carbon of the present invention has a relationshipbetween x and y in which y is 100×(1−0.8×1.45^((0.3-x))) or more in therange in which x is 0.5 or more and 20 or less, providing that theaverage particle diameter is x (MM) and the MS hardness is y (%), inother words, a is 1.45 or more in the above equation (I). In anactivated carbon of the present invention, practically a higher hardnessis preferable, and a in the equation (I) is preferably 1.60 or more.Meanwhile, the hardness is excessively increased, causing a problem thatit becomes difficult to be compatible with the adsorption performanceand it takes a long time to produce a spherical activated carbon.Therefore, a in equation (I) is preferably not more than 2.50, and morepreferably not more than 2.10.

The resent inventors have studied earnestly to increase the hardness ofa spherical activated carbon and succeeded in producing a sphericalactivated carbon having a high hardness which has not conventionallybeen produced from an infusible solid carbon material as a raw material.As a result of testing the resultant spherical activated carbons havingvarious hardness and particle diameters, the present inventors haveconfirmed that a spherical activated carbon in which the MS hardness andthe average particle diameter satisfy the above relationshipsignificantly reduces trouble caused by fracturing and abrasion when inuse in the applications described heretofore, compared to conventionalspherical activated carbons, and the present inventors have reached thepresent invention.

The average particle diameter of a spherical activated carbon of thepresent invention is one as measured by JIS K-1474. That is, the averageparticle diameter is calculated by classifying activated carbons byusing a screen specified by JIS and then by multiplying the weightfraction of the activated carbons classified by the median of theopening of the screen used for classification.

The particle diameter of a spherical activated carbon of the presentinvention is selected accordingly depending on the aspect of use.However, typically in the case where the spherical activated carbon isfilled in a container and used by ventilating, if the particle diameteris too large, the contacting efficiency between the gas and theactivated carbon is reduced, making it difficult to exhibit adsorptionability. Therefore, the average particle diameter is preferably 5.0 mmor less and more preferably 3.0 mm or less. Meanwhile, if the particlediameter is too small, the pressure loss tends to decrease in a statewhere a fluid is circulated. Accordingly, the average particle diameteris preferably 0.5 mm or more and more preferably 0.8 mm or more.

In general, if the hardness of an activated carbon is increased, theadsorption performance tends to decrease. The adsorption performance forthe application for prevention of automobile fuel evaporation isrepresented by the benzene adsorption amount. Here, the benzeneadsorption amount is measured in accordance with JIS K1474 of themeasurement of adsorption performance of solvent evaporation and isrepresented by the benzene amount (% by weight) adsorbed to the weightunit of the activated carbon in the equilibrium adsorption at aconcentration of one tenth of the saturated concentration. If thebenzene amount adsorbed is too small, the adsorption ability oftenpractically becomes insufficient, and if the benzene amount adsorbed ismore than a given amount, it becomes increasingly difficult to increasethe hardness. Therefore, the benzene amount adsorbed is preferably 25 to65%. However, in the case of an activated carbon used for gas separationby using a pressure swing system gas separation apparatus, the benzeneamount adsorbed may be lower than 25% because the molecule to beadsorbed is smaller than that of benzene.

A spherical activated carbon of the present invention may be subjectedto chemical and physical treatments where necessary. As such surfacemodifications, there may be mentioned impregnation of a salt of metalsuch as silver and iron, an oxide and a mineral acid. Further, thespherical activated carbon may contain on the surface and/or insideother fine particles in the range where the inherent function ofactivated carbon is not impaired. As examples of these substances, theremay be mentioned metal oxides such as silica, alumina, zeolite and thelike.

Hereinafter, a method of producing a spherical activated carbon of thepresent invention is explained. The spherical activated carbon in thepresent invention may be produced by a process comprising mixing aninfusible solid carbon material (hereinafter, may be abbreviated as araw material carbon material) as a raw material with a carbonizablebinder and water where necessary, extruding the mixture into a strandform; cutting the resultant strand to a suitable size and then carryingout rolling granulation so as to obtain a spherical form; infusibilizingthe same under appropriate conditions corresponding to the particlediameter; and performing infusibilization, carbonization and activationunder conditions appropriately restricting the contact between theformed mixture and a gas phase portion.

The raw material carbon material used here is not particularly limitedas long as it is an infusible solid carbon material as mentioned above.However, coal and coconut shell charcoal are preferably used in view ofthe available readiness and the producibility of an activated carbonhaving various pores. Especially, coconut shell charcoal is favorablyused because it contains no hazardous impurities and an activated carbonhaving a wide range of performance may be produced from it.

The particle size of the raw material carbon material may be selecteddepending on the purpose of use, however, if the particle diameter istoo large, it becomes difficult to harden the carbon material, and thepore of the resultant spherical activated carbon becomes large, makingit difficult to increase the hardness. Meanwhile, the particle diameteris too small, the operating efficiency in forming the carbon material isdecreased. Therefore, as for the particle diameter of the raw materialcarbon material, the central particle diameter is preferably 1 μm to 100μm and more preferably 5 μm to 20 μm.

As the carbonizable binder, there may be mentioned an organic materialhaving a high boiling point such as coal tar, pitch, a thermosettingphenol resin and the like. The types and quantity of the binder areadjusted so that a raw material mixture is suitably softened at atemperature at which a raw material mixture is easily operated. In thisviewpoint, the binder is softened preferably in the range of 40° C. to100° C. Further, the quantity used of the carbonizable binder ispreferably 20 to 60 parts by weight and more preferably 35 to 55 partsby weight with respect to 100 parts by weight of the carbon material.

In addition to the carbon material and the carbonizable binder, water isadded if necessary. The addition amount of water is varied with the kindand particle diameter of the raw material carbon material and the typeof the binder, and approximately 5 to 30 parts by weight of which ispreferably added with respect to 100 parts by weight of the carbonmaterial, in order to make it possible to facilitate the extrusion ofthe mixture in extruding to a strand and then obtain an excellentformability in carrying out rolling granulation.

In addition to the carbon material, carbonizable binder and water, otheradditives may be added as long as the inherent function of activatedcarbon of the present invention is not impaired. As such additives,there may be exemplified by an alkali metal compound such as lithium,sodium, potassium and the like, an alkaline earth metal compound such asmagnesium, calcium and the like, other typical metals such as silicone,aluminum and the like and compounds thereof, a transition metal such astitanium, iron, copper, silver, zinc and the like and a compoundthereof, and a complex oxide such as silica alumina, zeolite, activatedwhite clay, clay and the like, which are added to improve functions, forexample, to improve adsorption performance and provide catalyticperformance. The amount of additives except for the binder andcarbonizable binder, which is satisfactory to a degree that the functionof the activated carbon is not impaired, is preferably not more than 30parts by weight and more preferably not more than 10 parts by weightwith respect to 100 parts by weight of the raw material carbon material.

The above-mentioned raw material carbon material, carbonizable binder,and water and other additives where necessary to are blended produce acarbon mixture. The conditions and blending device are determinedaccordingly depending on the type and composition of the carbon materialand carbonizable binder. Conventionally well-known various blendingmachines may be adopted as a blending device, however, for example,there may be exemplified by a two-axis kneader blending machine, aone-axis kneader blending machine and the like. The temperature at thetime of blending, which is not particularly limited as long as it is atemperature at which the binder maintains appropriate fluidity, istypically preferably 20 to 100° C. and more preferably 40 to 80° C.

The above-mentioned carbon material mixture obtained by mixing andblending is extruded into a strand form and cut to form a pellet havingan appropriate diameter. This process is, for example, carried out by apellet mill and the like. The nozzle bore diameter and cutting diameterare set depending on the targeted diameter of a spherical activatedcarbon. In obtaining a spherical activated carbon having a high hardnessand high bulk density, it is important that a strand is thus formed oncewithout directly forming the mixture to a spherical form. Although thereason is not necessarily clarified, it is estimated that a relativelylarge air bubble and variation in the composition, which are responsiblefor the cause of the structural defect in changing the carbon materialmixture to the activated carbon, are solved by once blending andextruding the mixture. Further, it is important to cut the mixture as astrand once in order to obtain a product having a small particlediameter distribution, compared to a method of directly carrying outrolling granulation of a powder raw material and a binder.

The cut strand is formed to a spherical form by a method of rollinggranulation and the like. In the case where rolling granulation isperformed, a typical granulation apparatus may be used. As suchapparatuses, there may be exemplified, for example, Malmelizermanufactured by Dalton Co., Ltd., High-speed Mixer manufactured by FukaePowtec Co., Ltd. and the like. The temperature of rolling granulation isnot specifically limited, however, the rolling granulation is carriedout preferably at a temperature in the range of 40 to 100° C. becausethe temperature adjustment with a granulation machine is easy.

The spherical carbon material mixture obtained by forming a strand to aspherical form by the above-mentioned methods is formed as a sphericalactivated carbon through a process such as infusibilization,carbonization and activation and the like. In order to obtain anactivated carbon having a high hardness, all these processes conditionsare required to be suitably adjusted. Since the suitable conditions forobtaining a spherical activated carbon of the present invention arevaried with the particle diameter of a spherical carbon materialmixture, the type of a raw material carbon material and the type andamount used of a carbonizable binder and the like, they are generallydifficult to be defined. However, in any processes, if the conditionsare adjusted to the direction of suppressing the contact between thecarbon material mixture and a gas, there is a tendency that an activatedcarbon having a high hardness is easily obtained.

The spherical carbon material mixture obtained by forming a strand to aspherical form is infusibilized under an atmosphere containing oxygen.Here, an atmosphere containing oxygen is referred to as ordinary air, ora mixed gas of oxygen and nitrogen, or a gas containing moisture, carbondioxide and oxygen, and the like. Here, in order to increase thehardness of the finished product, it is desirable that the oxygenconcentration, temperature, the situation and time of contacting with agas are suitably adjusted depending on the particle diameter. Thecondition of infusibilization is adjusted so that an adequate oxidationvelocity corresponding to the particle diameter of the spherical carbonis obtained. However, the infusibilization is typically preferablycarried out at a temperature of not more than 400° C. and under anoxygen concentration of 5 to 22%. At this time, a suitable restrictionof the contact with a gas is effective for the increase in hardness.Depending on the conditions of the spherical activated carbon materialmixture, it is desirable that the contact with a gas is restricted sothat suitable infusibilization may be typically carried out byperforming the infusibilization for more than one hour including thetemperature rising time. Here, as an apparatus used forinfusibilization, one which is typically well-known may be used,however, there is preferably used a moving layer type device, forexample, a rotary kiln, a Herreshoff type multistage floor furnace and asleep furnace from the viewpoint that the contact with a gas may beeasily controlled.

The spherical carbon infusibilized is subjected to carbonizationtreatment in an inert gas. The suitable conditions of carbonization areselected depending on the particle diameter, however, the temperature ispreferably increased to a temperature approximately between 500 and 700°C. Here, an inert gas is referred to as a gas which is inert toward acarbon material in the above range of the temperature, typically meansnitrogen and may be allowed to contain other nonoxidizable gases. Sincethe binder is carbonized by the infusibilization and carbonizationtreatment, the spherical activated carbon which is finally obtained doesnot substantially contain the binder. For the carbonization, theabove-mentioned well-known devices which are usually used may be used.

The spherical carbon obtained by carbonization is further activated inan activation gas atmosphere, thereby there may be obtained a sphericalactivated carbon of the present invention having a high hardness and isexcellent in prevention of dust generation. In order to obtain anactivated carbon having a high hardness, it is desirable that thecontact between a spherical carbon and the activation gas atmosphere issuitably restricted at the time of activation. For that purpose, theremay be preferably used a moving layer type device such as a rotary kiln,a Herreshoff type multistage floor furnace and a sleep furnace. Thesuitable conditions of activation are required to be selected dependingon the particle diameter, however, a temperature approximately between800 and 1000° C. is preferably adopted. Here, an activation gasatmosphere referred to as moisture, carbon dioxide and a mixed gasthereof. As the activation gas atmosphere, there is preferably used anoil burning mixed gas which has a high moisture content and containscarbon dioxide. Here also, in order to produce a given activated carbonby controlling the contact with a gas, the activation is carried outpreferably for more than 3 hours, and more preferably for more than 5hours. Meanwhile, there is no problem to carry out the activation for aprolonged period of time by strictly restricting the contact with a gas,from a view point of obtaining an activated carbon having a highhardness. However, the activation time is practically preferably 60hours or less because the production efficiency is reduced.

Since a spherical activated carbon of the present invention generatesless dust when oscillation is applied or when it contacts with a highvelocity gas, it is used for prevention of automobile fuel evaporation,and the like. Further, it is used for absorbing hazardous substances ina clean room for producing drugs and the like, is used for absorbinghazardous substances inside or in the vicinity of precision equipmentand an electronic device which dislike dust, for example, for absorbingsubstances which exert an influence on a hard disk of computer and thelike, and is used for the treatment of a constant or higher flow rate ofa gas, for example, a pressure swing system gas separation device andthe like.

Hereinafter, the present invention is explained with reference toExamples, but the present invention is not limited to these Examples.

EXAMPLE 1

Coconut shell charcoal (carbon content: 85%) was pulverized with apulverizer to obtain carbon particles having a particle diameter of 200meshes or less (corresponding to a particle diameter of 75 μm). Thecentral particle diameter of the resultant fine powders of the coconutshell charcoal was 10 μm. To 100 parts by weight of the fine powders ofthe coconut shell charcoal were added 40 parts by weight of coal tar(carbon content: 60%) and 10 parts by weight of water, and the mixturewas kneaded with Versatile Mixing and Blending Device, 30DM Type (brandname) manufactured by Dalton Co., Ltd. at a revolution speed of 68 rpmfor 20 minutes. The resultant mixture was extruded with a pellet mil(Type 2 (brand name) manufactured by Jyoda Iron Works Co., Ltd. into astrand form, which was cut to obtain an extruded material in a pelletform having a diameter of 1.6 mm and a length of 1.5 to 5 mm. Theextruded material was formed by using High-speed Mixer FS-G Type (brandname) (volume:10 liters, diameter: 400 mm) manufactured by Fukae PowtecCo., Ltd. at 60° C. and at a revolution speed of 400 rpm) for 10 minutesto obtain a formed material in a spherical form having an averageparticle diameter of 2.3 mm.

The resultant spherical formed material was heated to 200° C. over 30minutes by using a rotary kiln (diameter: 600 mm) at a revolution speedof 4 rpm and under an atmosphere of air and was infusibilized for 45minutes and subsequently the resultant material was carbonized byheating to 600° C. over 60 minutes under an atmosphere of an inert gasin the same furnace. Further, the carbonized material was activated witha nitrogen gas and steam (the steam partial pressure: 49%) at 900° C.for 20 hours to obtain an spherical activated carbon having an averageparticle diameter of 1.8 mm.

The MS hardness of the resultant spherical activated carbon was 63.3%.Since the particle diameter x equals 1.8 mm, 100×(1−0.8×1.45^((0.3-x)))is 54.2, which value is exceeded by the MS hardness of this sphericalactivated carbon. Further, the benzene adsorption amount of thisspherical activated carbon was 41.5%, the bulk density was 0.52 g/ml,and the ratio of long radius to short radius was in the range of 1 to1.5.

EXAMPLE 2

A process until the kneading is completed was performed under the sameconditions as the above-mentioned Example 1, and the resultant mixturewas extruded by a pellet mill into a strand form and the strand was cutto obtain an extruded material in a pellet form having a diameter of 3.5mm and a length of 3 to 9 mm. The extruded material was treated underthe same conditions as Example 1 to obtain a spherical activated carbonhaving an average particle diameter of 4.5 mm.

The MS hardness of the resultant spherical activated carbon was 91.9%.Since x equals 4.5 mm, 100×(1−0.8×1.45^((0.3-x))) is 83.2, which valueis exceeded by the MS hardness of this spherical activated carbon.Further, the benzene adsorption amount of this spherical activatedcarbon was 43.0%, the bulk density was 0.54 g/ml, and the ratio of longradius to short radius was in the range of 1 to 1.5.

EXAMPLE 3

A process until the kneading was performed under the same conditions asthe above-mentioned Example 1, and the resultant mixture was extruded bya pellet mill into a strand form and the strand was cut to obtain anextruded material in a pellet form having a diameter of 0.8 mm and alength of 1 to 3 mm. The extruded material was treated under the sameconditions as Example 1 to obtain a spherical activated carbon having anaverage particle diameter of 1.1 mm. The MS hardness of the resultantspherical activated carbon was 54.6% and the benzene adsorption amountwas 41.6%. Since x equals 1.1, 100×(1−0.8×1.45^((0.3-x))) is 40.6, whichvalue is exceeded by the MS hardness of this spherical activated carbon.Further, the bulk density of this spherical activated carbon was 0.56g/ml and the ratio of long radius to short radius was in the range of 1to 1.5.

EXAMPLE 4

A spherical activated carbon was produced in the same manner as Example1 except that a coal powder (anthracite which was produced to have 2.5%by weight of ash by washing (button index: 0, fixed carbon content: 85%by weight, particle diameter: not more than 75 μm, central particlediameter: 10 μm) is used in place of a coconut shell charcoal and theactivation time was changed to 23 hours. However, the average particlediameter was adjusted to be 3.0 mm. The MS hardness of the resultantspherical activated carbon was 75.0%. Since x equals 3.0,100×(1−0.8×1.45^((0.3-x))) is 70.7, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.39 g/ml and the benzene adsorption amount was 58.6%.

Moreover, the butane working capacity (hereinafter referred to as BWC)of the spherical activated carbon was 14.6 g/100 ml as measured by ASTMD5228, which is an evaluation method of an activated carbon used forprevention of automobile fuel evaporation.

EXAMPLE 5

A process until the kneading was performed under the same conditions asthe above-mentioned Example 1, and the resultant mixture was extruded bya pellet mill into a strand form and the strand was cut to obtain anextruded material in a pellet form having a diameter of 2.3 mm and alength of 2 to 7 mm. The extruded material was formed by usingHigh-speed Mixer FS-G Type (brand name) (volume:10 liters, diameter: 400mm) manufactured by Fukae Powtec Co., Ltd. at 60° C. and at a revolutionspeed of 400 rpm for 10 minutes to obtain a formed material in aspherical form having an average particle diameter of 3.3 mm.

The resultant spherical formed material was heated to 200° C. over 30minutes by using a rotary kiln (diameter: 600 mm) at a revolution speedof 4 rpm and under an atmosphere of air and infusibilized for 45 minutesand subsequently the resultant material was carbonized by heating to600° C. over 60 minutes under an atmosphere of an inert gas in the samefurnace. Further, the carbonized material was activated with a nitrogengas and steam (the steam partial pressure: 49%) at 900° C. for 20 hoursin a rotary kiln (400 mm in diameter) to obtain an spherical activatedcarbon having an average particle diameter of 2.6 mm.

The MS hardness of the resultant spherical activated carbon was 69.4%and the benzene adsorption amount was 42.1%. Since x equals 2.6,100×(1−0.8×1.45^((0.3-x))) is 66.0, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.51 g/ml and the ratio of long radius to short radius was in therange of 1 to 1.5.

EXAMPLE 6

The spherical formed material having an average particle diameter of 3.3mm, which was obtained under the same conditions as Example 5, wasinfusibilized and carbonized under the same conditions as Example 5 andsubsequently the carbonized material was activated under the sameconditions as Example 5 for 23 hours to obtain a spherical activatedcarbon having an average particle diameter of 2.5 mm. The MS hardness ofthe resultant spherical activated carbon was 68.2% and the benzeneadsorption amount was 54.6%. Since x equals 2.5,100×(1−-0.8×1.45^((0.3-x))) is 64.7, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.44 g/ml and the ratio of long radius to short radius was in therange of 1 to 1.5.

EXAMPLE 7

The spherical formed material having an average particle diameter of 3.3mm, which was obtained under the same conditions as Example 5, wasinfusibilized and carbonized under the same conditions as Example 5 andsubsequently the carbonized material was activated under the sameconditions as Example 5 for 25 hours to obtain a spherical activatedcarbon having an average particle diameter of 2.5 mm. The MS hardness ofthe resultant spherical activated carbon was 65.7% and the benzeneadsorption amount was 65.2%. Since x equals 2.5,100×(1−0.8×1.45^((0.3-x))) is 64.7, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.40 g/ml and the ratio of long radius to short radius was in therange of 1 to 1.5.

COMPARATIVE EXAMPLE 1

A spherical activated carbon was produced in accordance with theexamples described in Patent Reference 4 (Published Examined PatentApplication No. Sho 46-41210). Weak coking coal having an ash of 3% wasused as raw material coal, dried to the water content of 2% andsubsequently pulverized to 100 meshes or less. To the resultant fineparticle coal was added 20% by weight of pulp effluent separatelyprepared as coking coal based on raw material coal, and water was alsosecondarily added to adjust to a water content of 20%. The coal wasadjusted to a water content of 12 to 15% in the examples of PatentReference 4, however, coal having this range of the water content couldnot be formed to a spherical form. The mixture was thoroughly blendedand was formed by using High-speed Mixer FS-G Type (volume:10 liters,diameter: 400 mm) manufactured by Fukae Powtec Co., Ltd. at 35° C. andat a revolution speed of 100 rpm for 10 minutes to obtain a sphericalformed material having an average particle diameter of 2.3 mm. Theresultant formed material was dried at 100° C., modified at 360° C. andthen sintered at 530° C. to produce a carbon material which is suitablefor carbonization. The resultant carbon material was carbonized at 900°C. in a rotary kiln, and further activated by steam for 2 hours by afluidized-bed activation furnace under the conditions of 900° C. and asteam partial pressure of 40%. The average particle diameter of theresultant activated carbon was 1.8 mm.

The MS hardness of the resultant spherical activated carbon was 46.0%and the benzene adsorption amount was 32.2%. Therefore, since x equals1.8, 100×(1−0.8×1.45^((0.3-x))) is 54.2, which value is exceeded by theMS hardness. Further, the bulk density of this spherical activatedcarbon was 0.47 g/ml and the ratio of long radius to short radius was inthe range of 1 to 1.5. Meanwhile, Patent Reference 4 describes anactivated carbon having a targeted product particle diameter of 3 to 10mm, the MS hardness of 90%, a benzene adsorption amount of 30%. Althougha particle diameter is not described in the above publication, assumingthat the average particle diameter in the vicinity of center is 7.0 mm,100×(1−0.8×1.45^((0.3-x))) is 93.4, which value is exceeded by the MShardness. Moreover, since the value of a in equation (I),y=100×(1−0.8×a^((0.3-x))) of the spherical activated carbon ofComparative Example 1 corresponds to 1.34, and that of the activatedcarbon of Patent Reference 4 corresponds to 1.36 assuming that theaverage particle diameter is 7.0 mm, it may be said that the essentialharness of the activated carbon of Comparative Example 1 and that of theactivated carbon described in the above publication are almost the same.

COMPARATIVE EXAMPLE 2

A spherical activated carbon was produced in the same manner as Example1 except that the conditions of infusibilization was changed to 250° C.and 2 hours, and the activation was performed for 2 hours under theconditions of 850° C. and a steam partial pressure of 40% by using afluidized-bed activation furnace. The average particle diameter of thespherical activated carbon was 2.0 mm, the MS hardness was 52.4% and thebenzene adsorption amount was 38.2%. Since x equals 2.0,100×(1−0.8×1.45^((0.3-x))) is 57.5, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.49 g/ml and the ratio of long radius to short radius was in therange of 1 to 1.5.

COMPARATIVE EXAMPLE 3

As a commercially available spherical activated carbon, the physicalproperties were measured for X-7000 (brand name) produced by JapanEnviroChemicals, Ltd., which had an average particle diameter of 1.6.The MS hardness was 28.6%, and since x equals 1.6,100×(1−0.8×1.45^((0.3-x))) is 50.7, which value is exceeded by the MShardness. Further, the benzene adsorption amount was 31.6%

COMPARATIVE EXAMPLE 4

A spherical activated carbon was produced in accordance with theexamples described in Patent Reference 3 (Published Unexamined PatentApplication No. Hei 03-030834). Bituminous coal was used as raw materialcoal, dried to the water content of 2% and subsequently pulverized to100 meshes or less. To the resultant fine particle coal were added 12parts by weight of pulp effluent separately prepared as coking coal andsimultaneously 8 parts by weight of water with respect to 100 parts byweight of raw material bituminous coal, and subsequently the mixture wasblended by a batch type kneader to obtain a raw material carbon materialmixture. Further, while water was being added to the mixture, themixture was formed by using High-speed Mixer FS-G Type (brandname)(volume:10 liters, diameter: 400 mm) manufactured by Fukae PowtecCo., Ltd. at 40° C. and at a revolution speed of 100 rpm for 10 minutesto obtain a spherical formed material having an average particlediameter of 2.0 mm. The amount of water added was 25 parts by weight intotal with respect to 100 parts by weight of raw material bituminouscoal. The resultant spherical formed material was dried at 100° C., andcarbonized by heating from 250° C. to 600° C. at a rate of 3.5° C./minunder a flow of nitrogen gas in a rotary kiln (diameter: 600 mm). Theresultant carbonized product was activated by steam (steam partialpressure: 49%) in a rotary kiln (diameter: 400 mm) at 900° C. Theaverage particle diameter of the resultant activated carbon was 1.7 mm.

The MS hardness of the resultant spherical carbon was 37.3% and thebenzene adsorption amount was 27.5%. Therefore, since x equals 1.7,100×(1-0.8×1.45^((0.3-x))) is 52.4, which value is exceeded by the MShardness. Further, the bulk density of this spherical activated carbonwas 0.53 g/ml and the ratio of long radius to short radius was in therange of 1 to 1.5.

REFERENCE EXAMPLE 1

The powdering percentage was measured for the activated carbons ofExamples 1 to 7 and Comparative Examples 1 to 3. The powderingpercentage referred here is represented by the concentration, which isobtained by putting 0.1 g of predried spherical activated carbon into a100 ml conical flask equipped with a ground glass stopper, and shakingthe flask at 200 rpm for 3 hours and then adding 25 ml of ethanol andshaking the flask at 140 rpm for 30 minutes and immediately taking outthe suspension and measuring the absorbance at 650 nm by aabsorptiometer, and finally converting the absorbance into theconcentration of the suspension using standard curves prepared inadvance. The above-mentioned powdering percentage becomes an index ofdust generation property in the case where an activated carbon is usedin a device for prevention of automobile fuel evaporation (a canister)and the like.

These results are shown together with the results of Examples andComparative Examples in Table 1. Incidentally, in the Examples 1 and 3,since the concentration of the suspension exceeds the upper limit of theabsorbance measurement in the above-mentioned measuring method, themeasurement was performed by further diluting the suspension byten-fold. In addition, the value of a is also described when therelationship between the average particle diameter x and the MS hardnessy is substituted to the relationship equation,y=100×(1−0.8×a^((0.3-x))). It is indicated that, as the MS hardnessincreases, the powdering percentage is improved, and in the Exampleswhich exceed a=1.45, the powdering percentage which is not more than 1%is within a range having no practical problems. TABLE 1 Average BzParticle MS Adsorption Bulk Powdering JIS Activated Diameter Hardness aAmount density Percentage Hardness carbon (mm) (%) value (%) (g/cc) (%)(%) Example 1 1.8 63.3 1.68 41.5 0.52 0.56 99.8 Example 2 4.5 91.9 1.7343.0 0.54 0.80 99.8 Example 3 1.1 54.6 2.03 41.6 0.58 0.45 99.8 Example4 3.0 75.0 1.54 58.6 0.39 0.65 99.8 Example 5 2.6 69.4 1.52 42.1 0.510.87 99.8 Example 6 2.5 68.2 1.52 54.6 0.44 0.71 99.8 Example 7 2.5 65.71.47 65.2 0.40 0.59 99.8 Comparative 1.8 46.0 1.34 32.2 0.47 2.40 92.0Example 1 Comparative 2.0 52.4 1.36 38.2 0.49 1.24 99.2 Example 2Comparative 1.6 28.6 1.09 31.6 0.45 2.98 95.3 Example 3 Comparative 1.737.3 1.19 27.5 0.53 2.40 99.2 Example 4

INDUSTRIAL APPLICABILITY

An activated carbon of the present invention is suitably used for adevice for prevention of automobile fuel evaporation (a canister), apressure swing system gas separation and the removal of hazardoussubstances under the environment which cannot allow dust and the like.

1: A spherical activated carbon produced from an infusible solid carbonmaterial as a raw material, characterized in that providing that x (mm)represents an average particle diameter and y (%) an MS hardness, in therange in which x is 0.5 or more and 20 or less, y is100×(1−0.8×1.45^((0.3-x))) or less. 2: The spherical activated carbonaccording to claim 1, characterized in that the infusible solid carbonmaterial is at least one selected from coconut shell charcoal and coal.3: The spherical activated carbon according to claim 1, characterized inthat an average particle diameter of the spherical activated carbon is0.5 mm or more and 5.0 mm or less. 4: The spherical activated carbonaccording to claim 1, characterized in that a benzene adsorption amountof the spherical activated carbon is 25% or more and 65% or less. 5: Thespherical activated carbon according to claim 1, characterized in thatthe spherical activated carbon is an activated carbon used forprevention of automobile fuel evaporation. 6: A method of producing thespherical activated carbon according to claim 1, comprising the stepsof: once extruding a mixture containing 100 parts by weight of aninfusible solid carbon material and 20 to 60 parts by weight of acarbonizable binder to a strand form; cutting the strand and thencarrying out rolling granulation so as to obtain a spherical form;infusibilizing the same under an atmosphere having an oxygenconcentration of more than 5% to 22% at 400° C. or less; performingcarbonization under an atmosphere of inert gas at a temperature of 500to 800° C.; and performing activation treatment in an atmosphere havinga steam partial pressure of 10 to 70% at a temperature of 800 to 1000°C.